This invention is generally related to seals for use in storage tanks and is particularly directed to a combination weather and vapor seal for covering the rim space between the inner wall of a storage tank and its floating roof.
In the storage of petroleum products such as gasoline, it is conventional to store the product in a cylindrical tank whose roof floats on the contained product. As the level of the product rises and falls, the floating roof undergoes a corresponding movement.
To enable the roof to rise and fall freely with the level of the contained product, the diameter of the roof is made smaller than the diameter of the tank so that the roof is actually situated within and spaced from the walls of the tank. The space which surrounds the roof and which separates it from the walls of the tank is commonly known as the "rim space". Unless the rim space is otherwise covered, the product contained in the tank is exposed to the atmosphere wherever the rim space exists. As a result of such exposure, the product may be contaminated by rainfall and other airborne contaminants. In addition, vapors from the contained product may escape through an uncovered rim space, thereby depleting the product and presenting a possibly hazardous condition.
In the past, many efforts have been made to provide a seal for the rim space which will effectively seal in vapors and seal out atmospheric contaminants. Such a seal typically extends between the inner tank wall and the top of the roof and is mounted so as to ride up and down with the movement of the roof. One of the most practical of the prior art seals has included a flexible metal shield which is mounted to the upper perimeter of the roof and which extends diagonally upwardly toward the inner wall of the tank. A rubber wiper blade has been mounted on the end of the shield for engaging and pressing against the inner wall of the tank.
Although the above-described seal has been found to be an improvement over other seals, it has also been found to be deficient in two regards. Specifically, when such a seal has been used with storage tanks having a variable rim space dimension, effective contact between the wiper blade and the inner tank wall is lost when the roof moves laterally so as to increase the rim space on one side of the roof and decrease the rim space on an opposed side of the roof. On the side of the roof where the rim space increases, either insufficient contact or no contact may result between the tank wall and the wiper blade. In addition, when the wiper blade makes minimum contact with the tank wall and rises upwardly with the roof, engagement between the wiper blade and a protruding weld seam in the tank wall can cause the contacting edge of the wiper blade to momentarily stop at the weld seam while the remainder of the seal continues upwardly. As a result, the metal shield may be bent downwardly and be permanently deformed.
A second deficiency found to exist in the above-described seal is that vapors from the contained product may escape to the atmosphere even when the wiper blade is making firm contact with the wall of the tank. Such vapor loss may occur because of the fact that the metal shield is not a unitary continuous piece that extends completely around the rim space. Rather, it is made of a plurality of adjacent shield sections which lap or abut each other. The interfaces between adjacent sections of the shield thus provide escape routes for the vapor to the atmosphere. Even though such vapor loss is not great, the tendency of environmental standards is to prohibit even small vapor losses.
An additional problem with the above-described seal is that it permits the temperature within the rim space to reach an undesirably high level, thereby accelerating the generation of vapors from the product contained in the tank. As a consequence of the accelerated generation of vapor, more of the product may be depleted and more vapor may escape.
Accordingly, it is an object of this invention to provide an improved seal for the rim space of a storage tank which is not subject to the problems described above.
It is a more specific object of this invention to provide a rim space seal capable of making effective sealing contact between a wiper blade and an interior tank wall under conditions of varying rim space dimensions.
It is another object of this invention to provide a rim space seal which tends to maintain the temperature within the rim space at a relatively low level.